When smelting and purifying metals, gas is sometimes introduced into molten metal to remove impurities. Specifically, when processing molten aluminum, it is desirable to remove dissolved gases, particularly hydrogen, and to remove dissolved metals, particularly magnesium. Those skilled in the art refer to removing dissolved gas from molten aluminum as "degassing,"and refer to removing magnesium as "demagging." Nitrogen, argon or freon is generally released into molten metal for degassing purposes while chlorine gas is generally used for demagging.
When demagging or degassing aluminum, gas is released into a quantity of molten aluminum, this quantity generally being referred to as a bath of molten aluminum. The bath is usually contained within the walls of a reverbatory furnace. The present invention can be used for demagging or degassing or any application wherein gas is released into molten metal.
When demagging aluminum, chlorine is released into the bath and bonds, or reacts, with magnesium wherein each pound of magnesium reacts with approximately 2.92 pounds of chlorine to form magnesium chloride (MgCl.sub.2). Several methods for introducing chlorine into a molten aluminum bath are disclosed in the prior art. For example, it is known to introduce a flux containing chlorine into the bath, rather than introducing chlorine gas. Another method utilizes a gas-injection system including a pump having a discharge, a metal-transfer conduit attached to and extending from the discharge and a gas-injection conduit connected to the top of, and extending into, the metal-transfer conduit. Molten aluminum is pumped through the metal-transfer conduit and gas is injected through the gas-injection conduit into the upper portion of the metal-transfer conduit.
Other prior art includes: (a) a molten metal pump and gas-injection apparatus whereby gas is introduced through a tube into a passage and is released into molten metal entering the pump inlet; (b) a gas-treatment apparatus comprising: (i) a purification device, which is immersed in a molten metal bath contained within a furnace, and (ii) a decanting and degassing tank located outside of the bath; and (c) U.S. Pat. No. 5,662,725 to Cooper entitled "System And Device For Removing Impurities From Molten Metal," which discloses an apparatus that releases gas into the bottom or sides of a moving molten metal stream so as to better disperse the gas within the stream (the disclosure of U.S. Pat. No. 5,662,725 is incorporated herein by reference).
Specific examples of prior-art devices are disclosed in U.S. Pat. No. 3,650,730 to Derham et al., U.S. Pat. No. 3,767,382 to Bruno et al., U.S. Pat. No. 4,169,584 to Mangalick, U.S. Pat. No. 4,351,314 to Koch, U.S. Pat. No. 4,003,560 to Carbonnel, and U.S. Pat. No. 5,203,681 to Cooper.
One problem with the known gas-injection or gas-release devices (hereinafter collectively referred to as gas-release devices) is that they normally extend into either (a) a molten metal stream travelling through a metal-transfer device, such as a pump discharge or a metal-transfer conduit extending from the discharge, or (b) a flowing molten metal stream in the open molten metal bath. When the gas is introduced into any such molten metal stream it is swept downstream and the gas and molten metal are only confined in an enclosed space over a relatively short distance. Another problem with some of these devices is that they release gas in large bubbles near the inside, upper surface of a metal-transfer device. The gas becomes mixed with the molten metal only through the turbulent action of the flowing molten metal. Because of its buoyancy, much of the gas travels along the top of the metal-transfer device and never mixes with the molten metal.
Removing contaminant gases (known as degassing), such as hydrogen, dissolved in aluminum is usually done in an open metal bath separate and downstream from the charging furnace where the gas-injection or gas-release devices described above are used to demag the aluminum. Degassing is accomplished by the use of a rotary degasser of which there are several designs known to those skilled in the art. Some problems with rotary degassers are that they do not: (1) circulate the molten metal through an enclosed space such as a metal-transfer device, and (2) release gas into a defined stream of molten metal; instead, they release gas into the bottom of the molten metal bath. In addition to removing dissolved contaminant gases, such as hydrogen, degassing removes some solid impurities, such as oxides and salts, sodium fluoride, aluminum fluoride and other fluorides, which may be present in the molten metal suspension in the presence of dissolved hydrogen.
FIGS. 1A-1D represent known methods of releasing gas into (a) a metal-transfer device extending from a pump chamber, or (b) a molten metal stream exiting a metal-transfer device. FIG. 1A shows a pump casing C1 having a pump chamber CH1, a discharge D1 and a gas-release device G1 positioned in discharge D1. Gas (shown as small circles or bubbles) exits G1 into discharge D1 and is dispersed into the molten metal stream. FIG. 1B shows a pump casing C2 having a pump chamber CH2, a discharge D2 and a gas-release device G2 positioned in discharge D2. A metal-transfer conduit MC2 extends from discharge D2. Gas exits G2 into discharge D2 and is dispersed into the molten metal stream. The addition of metal-transfer conduit MC2 increases the distance and time that the metal and gas are confined in an enclosed space. FIG. 1C shows a pump casing C3 having a pump chamber CH3 and a discharge D3. A gas-release device G3 is positioned immediately outside of discharge D3 and releases gas into the molten metal stream exiting discharge D3. FIG. 1D shows a pump casing C4 that includes a pump chamber CH4 and a discharge D4. A metal-transfer conduit MC4 extends from discharge D4. A gas-release device G4 extends into metal-transfer conduit MC4 and releases gas therein. The gas mixes with the metal stream moving through conduit MC4. For each of these known methods, the gas and metal are either dispersed (a) throughout only part of the length of the metal-transfer device (or the combined length of the two metal-transfer devices for a structure such as the one shown in FIG. 2), or (b) not confined at all within an enclosed space.
As will be appreciated by those skilled in the art, the greater the dispersion of gas within the molten metal stream, and/or the longer the gas and metal arc confined, the greater the reaction between the impurities in the metal and the gas.
Improving the efficiency of the demagging and/or degassing process is highly desirable. It reduces material costs because less gas is used. Furthermore, chlorine gas that does not bond with magnesium to form MgCL.sub.2 either bonds with aluminum to form aluminum trichloride, an undesirable contaminant, or rises to the top of the molten metal bath and escapes into the atmosphere, where it is an undesirable pollutant. If dissolved contaminant gases are not removed, the resulting aluminum products will contain entrapped gas forming small cavities or pockets. Products formed with these small gas pockets are undesirable because they may have uneven surfaces, contain holes or lack structural integrity.